The invention described herein arose at the Lawrence Berkeley Laboratory in the course of, or under, Contract No. W-7405-ENG-48 between the U.S. Department of Energy and the University of California.
The invention relates to determining apparent tensile strength of a material, particularly to determining the tensile strength of rock, and more particularly to method and apparatus for determining the tensile strength and/or the deformation modulus of rock by rubber fracturing.
In a majority of rock mechanics problems, the engineer is seldom concerned with the tensile strength of intact rock, for the inherent discontinuities are the predominant structural component that usually determines the strength of the rock mass. However, there are a limited number of important situations wherein the knowledge of the apparent tensile strength of intact rock is of fundamental importance. For example, a knowledge of the apparent tensile strength in a hydraulic fracture experiment for the determination of in situ stress is fundamentally necessary if the state of stress is to be determined from the initiation of the hydraulically induced fracture.
In certain underground situations, such as the apparent tensile strength of intact rock beams defined by jointing or bedding planes is important in determining required rock bolting. Proposed high technology uses of underground cavities such as for liquid petroleum gas (LPG) storage require a knowledge of apparent tensile strength of the material of the surrounding walls, particularly as it relates to cyclic loading and potential fatigue failure. Also the apparent tensile strength and or deformation modulus of rock is of use to those involved in the application of hydraulic fracturing for the determination of in situ stress or for recovery of natural gas or other hydro-carbon deposits in relatively impermeable formations.
When intact rock samples are taken into the laboratory and tested to determine tensile strength, three observations should be made:
(1) The apparent tensile strength depends upon the sample size (the larger the specimen, the smaller the strength).
(2) The apparent tensile strength depends upon the type of test being performed.
(3) With any given test and specimen size, a scatter (usually skewed) about the mean is obtained.
The first of the above-listed three observations (commonly referred to as the size effect) is also observed with respect to compressive strength and an apparent Young's Modulus, although to a lesser extent than with tensile strength. However, this first observation has prompted many investigators to recognize that the tensile strength of brittle rock measured at the usual laboratory scale is not a material property.
The second of the above-listed observations has been brushed aside by using different names to refer to the strength observed in different tests. For example, the apparent tensile strength in bending is referred to as the Modulus of Rupture. The tensile strength determined by indirect tension tests is often referred to with an adjective taken from the test; for example, the Brazilian tensile strength or the split cylinder tensile strength.
The third above-listed observation is often totally neglected in the reporting of test results. Scatter about the mean is often attributed to testing methodology and/or sample-to-sample inhomogenity. Thus, more often than not, the only result of the tensile testing may be the mean without the standard deviation or any of the other statistical moments.
The "direct" tensile strength test is effected by loading a cylindrical or prismatic specimen in tension to failure. The specimen should fall in plane tensile stress and, for homogeneous isotropic material, the plane of failure should be normal to the axis of the specimen. The precautions are twofold: first, the applied tensile load must be uniformly distributed over the end of the specimen and parallel to its axis; second, the method of holding the specimen must not produce significant lateral stress in the specimen. To minimize the effects of this, the central section of metal tensile strength specimens is usually machined to a smaller diameter. With rock, this is a difficult operation.
It is known that even small scratches on the surface of metal specimens will reduce the tensile strength appreciably and some investigators fine-grind or polish the cylindrical surface of rock specimens to minimize this effect. However, most rock contains planes of weakness, incipient cracks, or other mechanical defects, and failure usually occurs at the point of these defects, a factor that causes a comparatively large deviation in the tensile strength measurements from a group of specimens. Also, as the length of tensile strength specimens is increased, the probability of including a weaker defect is increased and, hence, the average tensile strength decreases with the size of test specimens.
As pointed out above, two most commonly used testing methods are: The Brazilian Test; and the Modulus of Rupture Test. In the Brazilian Test, a cylindrical test specimen is placed horizontally between the bearing plate of a testing machine and loaded to failure in compression. If a line load is applied in a plane passing through the diameter of the cylindrical specimen from both directions, a uniform tensile stress should develop across the plane. However, in addition to the tensile strength that develops across the diametrical plane, a vertical compressive stress occurs in the plane. This compressive stress causes high shear stresses and local crushing along the loading line. This failure has been solved by applying the load to a strip of desired width.
The Modulus of Rupture Test is a measure of the outer fiber tensile strength of a material. This property is determined by loading either cylindrical or prismatic specimens in a three-point loading device to failure. In the loading device used for testing prismatic specimens, two of the bearing edges must be self-aligning (pivoted) to permit uniform loading of the specimen surfaces when such are not exactly parallel. Self-aligning bearing edges are not required for testing cylindrical specimens. The ends of the specimens do not need to be surfaced. The specimen should be loaded to failure at a uniform rate of 500 psi/min.
Although the Modulus of Rupture Test is a measure of the outer-fiber tensile strength of a material, the value obtained by this procedure is higher than determined in the "direct" Test. This higher value is presumed to result from the fact that only a small area (or point) on the opposite side of the specimen directly under the point is subject to the maximum tensile strain. Hence, the probability of a defect occurring at or near this point is less than that for an equivalent defect occurring in the length of a tensile specimen.
Various types of apparatus have been developed for testing samples or specimens of different types of material. For example, U.S. Pat. No. 756,644 issued Apr. 5, 1904 to A. N. Johnson and U.S. Pat. No. 3,122,916 issued Mar. 3, 1964 to R. Sedlacek relate to insertion of a rubber member into a borehole of a specimen to be tested and inflating the rubber member causing fracturing of the specimen. Further, U.S. Pat. No. 3,111,840 issued Nov. 26, 1963 to F. R. Barnet et al, U.S. Pat. No. 3,792,608 issued Feb. 19, 1974 to T. A. Holm et al, and U.S. Pat. No. 3,934,464 issued Jan. 27, 1976 to C. R. McCauley teach testing of a specimen of material by applying pressure of various types to the specimen causing fracturing thereof.
While the prior known methods and apparatus for carrying out the prior methods have been effective, at least to a certain extent, to provide the tensile strength of various materials, such have not been effective in determining the apparent tensile strength of rock and/or the apparent deformation modulus of rock. Thus, a need has existed for a more effective method of providing desired information relative to rock formations.
Therefore, it is an object of this invention to provide a method and apparatus for effectively determining the tensile strength and/or deformation modulus of rock.
A further object of the invention is to provide a rubber fracturing method and apparatus for determining the apparent tensile strength and/or deformation modulus of rock.
Another object of the invention is to provide a method of testing the tensile strength of a rock specimen by inserting a non-inflatable, deformable member in a borehole in the specimen and applying pressure to the deformable member causing rupturing of the specimen.